Propene (C3H6), often also referred to as propylene, is one of the most important starting materials in the chemical industry. It serves as a starting material for the production of chemicals such as acetone, acrylic acid, propylene oxide and acrolein, and therefore represents one of the most important basic chemicals in the chemical industry. Propylene is used, moreover, for the production of polypropylene (PP) plastics. The demand for the basic propylene chemical is increasing worldwide, with propylene, just like ethylene, being produced usually from petroleum in a steamcracker in a ratio which is dependent on the process and on the feedstocks.
In order to obtain additional propylene, there are a series of processes in existence, such as the PDH process, which starts from propane as reactant. Since, however, the greatest fraction of propylene continues to be produced by steamcracking (around 70%), the tendency is to convert the C4 to C8 olefins produced in crackers or other petrochemical plants into additional propylene and also, in part, into ethylene.
This can be done, firstly, via the metathesis process, which is based on a synproportionation of ethylene and butylene. Disadvantages here are that it requires the build-up of ethylene production and that only certain C4+ olefin isomers can be converted.
Also possible is an olefin conversion in which C4+ olefins are reacted to give propylene. This cracking operation is accomplished by means of the Propylur or the OCP process, and is utilized in particular in order to utilize for propylene production the—comparatively low-value—C4+ olefins that are produced in a cracker plant. Because of the endothermic nature of the reaction, however, the temperature in the reactor drops with ongoing conversion, and so limits the achievable propylene yield.
Appropriate, lastly, is the methanol-to-propylene process (also MTP® process), in which methanol/dimethyl ether or else other oxygenates are converted to propylene over a usually zeolitic catalyst.
The MTP process customarily has a propylene yield of about 65% (mole-C). For example, DE 10 027 159 A1 describes an MTP process with two shaft reactors. In a first, heterogeneously catalysed process step, methanol vapour is reacted to give dimethyl ether. This dimethyl ether is then divided into two substreams, which are supplied to respective first and second shaft reactors, in which a propylene-containing product mixture is produced over a zeolitic catalyst. Also introduced into the second shaft reactor, moreover, is the product stream of the first shaft reactor. As a result, a comparatively high propylene fraction of up to 50 vol % is achieved. At the same time, the process is very economic, given replacement of expensive tubular reactors by comparatively cost-effective shaft reactors.
DE 10 2006 026 103 A1 describes another type of reactor for implementing an MTP process. In this case, gaseous oxygenates are reacted together with steam at 400 to 470° C. in a closed reactor having a plurality of trays, to give olefins. The individual trays are filled with a fixed bed of catalyst. Each tray is fed individually with water and dimethyl ether and/or with a methanol-containing liquid phase, which is sprayed through a plurality of nozzle tubes. In this way, the optimum operating conditions for a stream with this degree of conversion can be set in each tray.
DE 10 2009 031 636, finally, describes a process for preparing the required oxygenates, more particularly methanol and dimethyl ether, which is designed in such a way as to allow flexible switching between methanol purification and production of dimethyl ether.
The MTP process is also described comprehensively in DE 10 2005 048 931 A1. Olefins are produced from a reactant mixture comprising steam and oxygenates, such as methanol and/or dimethyl ether. This reactant mixture is converted in at least one reactor, through a heterogeneously catalysed reaction, into a reaction mixture comprising low molecular mass olefins and gasoline hydrocarbons. By means of a suitable separation approach, higher-value olefins, particularly the C5+ fraction, can be returned as a recycling stream at least partially into the reactor, and very largely converted into propylene, thereby raising the propylene yield. An attempt is made to remove the aromatics using a distillation column. This is necessary since otherwise the aromatics in the reactor react with short-chain olefins (ethylene, propylene and butylene) in a Friedel-Crafts alkylation and therefore diminish the propylene yield.
A disadvantage of this process is that the separation performance is not sufficient to achieve sufficient removal of the aromatics. A change to the distillation parameters, such as liquid-phase temperature or reflux rate, does not result in an improvement in the separation performance, since the boiling points of the components to be separated are too similar. A further hindrance is that the yield of aromatics fluctuates greatly in the MTP process. The aromatics fraction in the feed line to a corresponding separation means is typically between 1 to 50 wt %, in particular between 5 and 40 wt %. Accordingly, hydrocarbons, more particularly aliphatic hydrocarbons and especially olefins, having more than 7 carbon atoms (C7+) have been withdrawn from the process together with the aromatics.